In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (hereinafter also referred to as EL). Such a light-emitting element has a structure where a layer including a light-emitting substance is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes, light emission can be obtained from the light-emitting substance.
The light-emitting element using electroluminescence has great characteristics which are advantageous in that it can be manufactured to be thin and lightweight and has very high response speed, for example. There are various possible applications of such a self-luminous light-emitting element. For example, such a light-emitting element is preferably used for a flat panel display because of having characteristics such that the pixels have higher visibility as compared to a liquid crystal display and no backlight is required.
Since the light-emitting element can be formed into a film shape, surface light emission from a large area can be readily obtained. This characteristic is difficult to be obtained by using a point light source typified by an incandescent lamp or an LED (light emitting diode), or a linear light source typified by a fluorescent lamp. Therefore, the light-emitting element has a high utility value as a surface light source that can be applied to illumination or the like.
The light-emitting element using electroluminescence is roughly classified in accordance with whether it includes an organic compound or an inorganic compound as a light-emitting substance. Now a principle of light emission of the light-emitting element which includes an organic compound as a light-emitting substance will be described. First, by applying a voltage between a pair of electrodes of the light-emitting element, electrons and holes are separately injected from the pair of electrodes into a layer including a light-emitting organic compound. Those carriers (the electrons and holes) are recombined, and then the light-emitting organic compound is excited. The light-emitting organic compound emits light when it returns to a ground state from the excited state.
An EL layer of a light-emitting element using electroluminescence is very thin. The EL layer is so thin that a pair of electrodes of the light-emitting element is easily short-circuited in the case where a conductive foreign substance enters between the pair of electrodes of the light-emitting element. The short circuit results in failures such as breakages of the light-emitting element, deterioration of the light-emitting element due to heat generation, and increase of power consumption due to leakage current.
Therefore, a method and a device for detecting defects in a light-emitting element using electroluminescence and for irradiating the defects with laser light so that the defects are insulated have been proposed (e.g., Patent Document 1).
Meanwhile, the density of current flowing through the EL layer may be increased in order to increase emission luminance of a light-emitting element using electroluminescence. However, when the driving voltage is increased in order that large current flows, power consumption is increased more and more. Additionally, large current flowing through the EL layer causes the light-emitting element to deteriorate more quickly.
Therefore, a light-emitting element in which a plurality of EL layers are stacked is proposed. Patent Document 2 discloses a light-emitting element in which a plurality of light-emitting units (hereinafter in this specification, the light-emitting unit is also referred to as an EL layer) are included and the light-emitting units are each partitioned by a charge generation layer which supplies carriers to the EL layers, as well as electrodes. In this specification, the charge generation layer is assumed to be equivalent to the electrode, and a region which is interposed between the charge generation layers adjacent to each other or between the charge generation layer and one of the electrodes adjacent to each other is counted as one EL layer. More specifically, Patent Document 2 discloses a light-emitting element in which a charge generation layer formed using vanadium pentoxide is provided over a metal-doped layer which functions as an electron injection layer of a first light-emitting unit, and further a second light-emitting unit is provided over the charge generation layer. The light-emitting element disclosed in Patent Document 2 can emit light at a luminance higher than a light-emitting element provided with one EL layer, when current with the same current density is supplied to each of these light-emitting elements. Additionally, the light-emitting element disclosed in Patent Document 2 can emit light at the same luminance as the light-emitting element provided with one EL layer, with lower power consumption.